1. Field of the Invention
The present invention relates to distance relays for protection of transmission lines, and more particularly to a distance relay for protection of a transmission line, which is capable of estimating apparent impedances with respect to faults on the transmission line involving fault resistances, more particularly faults at local and remote ends of a protection zone of the transmission line, and compensating for the magnitudes and phases of the estimated apparent impedances, so as to minimize a reactance effect resulting in a mal-operation or non-operation of the distance relay.
2. Description of the Related Art
In general terms, with power system being more complicated in construction, larger in size and higher in voltage, associated protection systems have also become very complicated in construction. In this regard, provided that such a protection system cannot display its appropriate protection ability with respect to a fault occurring at some part of an associated power system, the resulting accident ripple range and strength will be very great. If a protection relay bearing a primary responsibility does not accurately detect a fault occurring at any one part of a power system, a power interruption zone will be wider according to the rules of protection coordination. All backup protection relays for transmission lines and most main protection relays therefor are directional distance relays that are generally degraded in detection performance upon occurrence of a resistance earth fault. Further, as a transmission system becomes more complicated in construction, the average length of a transmission zone through which heavy load current flows is shortened, thereby causing a distance relay to perform a mal-operation or non operation due to a reactance effect which is a mutual coupling effect of a fault resistance and load current. Such a mal-operation or non-operation of a distance relay resulting from a reactance effect has recently been highlighted as a bigger problem.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a distance relay for protection of a transmission line, which is capable of estimating an apparent impedance with respect to an accident on the transmission line involving a fault resistance, and compensating for the magnitude and phase of the estimated approximate impedance on the basis of a previously calculated current distribution factor, so as to minimize a mutual coupling effect of the fault resistance and a zero-sequence component and a load flow effect upon occurrence of a fault, thereby minimizing a reactance effect resulting in a mal-operation or non-operation of the distance relay.
In accordance with the present invention, the above and other objects can be accomplished by the provision of a method for controlling an operation of a distance relay for protection of a transmission line having a minimized reactance effect, the distance relay being adapted to, it a single-phase to earth fault or an phase to phase short fault occurs on the transmission line, calculate an apparent impedance, detect an apparent fault location as a result of the calculation and generate a trip signal if the detected apparent fault location belongs to a protection zone, the method comprising the steps of: a) receiving a voltage/current signal flowing through the transmission line; b) filtering the received signal; c) obtaining a power frequency component from the filtered signal; d) comparing a level of the obtained power frequency component with a first predetermined threshold value to determine whether it is greater than the first predetermined threshold value, so as to determine whether overcurrent flows through the transmission line; e) if the overcurrent is determined to flow through the transmission line at the step d), comparing the amount of zero-sequence current of the filtered signal with a second predetermined threshold value to determine whether it is greater than the second predetermined threshold value, so as to determine which one of the earth fault and short fault has occurred on the transmission line; f) if the amount of the zero-sequence current of the filtered signal is determined to be greater than the second predetermined threshold value at the step e), determining that the earth fault has occurred on the transmission line and calculating a first apparent resistance (Rapp) and a first apparent reactance (Xapp) of a first apparent impedance; g) obtaining a first apparent fault location on the basis of the first apparent reactance; h) calculating a zero-sequence current distribution factor (CDFSa0) at the first apparant fault location; i) calculating a first phase angle (xcex1) on the basis of the calculated zero-sequence current distribution factor; j) obtaining a first corrected impedance by correcting the first apparent impedance on the basis of the first phase angle; k) determining whether a difference between the first corrected impedance and the previous first apparent impedance is smaller than a third predetermined threshold value and returning to the step g) if the difference between the first corrected impedance and the previous first apparent impedance is determined not to be smaller than the third predetermined threshold value; l) if the amount of the zero-sequence current of the filtered signal is determined not to be greater than the second predetermined threshold value at the step e), determining that the short fault has occurred on the transmission line and calculating a second apparent resistance (Rapp) and a second apparent reactance (Xapp) of a second apparent impedance; m) obtaining a second apparent fault location on the basis of the second apparent reactance; n) calculating a positive-sequence current distribution factor (CDFSa1) at the second apparent fault location; o) calculating a second phase angle (xcex1) on the basis of the calculated positive-sequence current distribution factor; p) obtaining a second corrected impedance by correcting the second apparent impedance on the basis of the second phase angle; q) determining whether a difference between the second corrected impedance and a previous second apparent impedance is smaller than a fourth predetermined threshold value and returning to the step m) if the difference between the second corrected impedance and the previous second apparent impedance is determined not to be smaller than the fourth predetermined threshold value; r) determining whether the first or second corrected impedance belongs to the protection zone, if the difference between the first corrected impedance and the previous first apparent impedance is determined to be smaller than the third predetermined threshold value at the step k) or if the difference between the second corrected impedance and the previous second apparent impedance is determined to be smaller than the fourth predetermined threshold value at the step q); and s) returning to the step b) if it as determined at the step r) that the first or second corrected impedance does not belong to the protection zone, and generating the trip signal if it is determined at the step r) that the first or second corrected impedance belongs to the protection zone.
In a feature of the present invention, if a fault occurs on a transmission line, a distance relay determines whether the amount of zero-sequence current resulting from overcurrent exceeds a predetermined threshold value, so as to determine whether the occurred fault is an earth fault or a short fault. The distance relay obtains an apparent fault location and a current distribution factor by calculating an apparent resistance and reactance based on the earth fault or short fault, and then corrects an apparent impedance by calculating a phase angle from equations of the apparent impedance and a voltage at a location where the distance relay is installed. After correcting the apparent impedance, the distance relay determines whether a trip signal is required to be generated. If the trip signal generation is required, then the distance relay generates the trip signal.